1. Field of the Invention
The present invention relates to an inertial drive actuator which drives a moving body by inertia, with respect to a vibration substrate which is subjected to reciprocated drive by a moving means, and a position control method of the inertial drive actuator.
2. Description of the Related Art
When a drive pulse of a waveform formed of a gently rising part and a rapidly falling part is applied to an electromechanical transducer such as a piezoelectric element, at (in) the gently rising part of the drive pulse, the piezoelectric element is displaced by being extended gradually in a direction of thickness, and at the rapidly falling part, the piezoelectric element is displaced by being contracted rapidly. Given this, by using this characteristic, by applying a drive pulse of a waveform as mentioned above to the piezoelectric element, discharge and charge are repeated at different speeds, and vibrations are generated in a direction of thickness in the piezoelectric element, at different speeds in the piezoelectric element. By the vibrations generated, a driving member fixed to the piezoelectric element is let to make a reciprocating movement at different speeds, and a moving member (body) which is attached to the driving member is moved in a predetermined direction. An actuator carrying out the abovementioned function has been known. Moreover, a displacement amount detection method of detecting a position of the moving member of such actuator or a position of a lens holding frame which is attached to the moving member of such actuator has been disclosed in Japanese Patent Application Laid-open Publication No. 2006-171172.
A conventional position detection method and a positioning method will be described below by referring to FIG. 13. Here, FIG. 13 is a cross-sectional view showing a schematic structure of a lens unit according to the conventional displacement detection method. In this lens unit, a holding frame 218 which holds a lens 216 is fixed to one end of an outer cylinder 214, and a lens 220 is held by a reduced-diameter portion 214B formed at the other end. A lens holding frame 226 which holds a lens 228 is attached to a drive shaft 222 connected to a piezoelectric element 230. The lens holding frame 226 is guided in an optical axis O direction by a guide shaft 224. A magnetic pole 238 of N polarity or S polarity is polarized at a fixed interval alternately along a longitudinal direction of the drive shaft 222. Whereas, a magnetic resistive element 236 is fixed to the holding frame 226, and an MR sensor 234 is formed by the magnetic pole 238 and the magnetic resistive element 236. When the magnetic resistive element 236 is displaced along the drive shaft 222 due to a displacement of the holding frame 226, since the magnetic resistive element 236 intersects the leakage magnetic flux of the magnetic pole 238, a value of resistance of the magnetic resistive element 236 changes periodically, and generates a predetermined position detection pulse. By counting the number of these position detection pulses, it is possible to find an amount of displacement of the holding frame 226 and the lens 228 in a direction of arrow A and a direction of arrow B. Moreover, in the outer cylinder 214, a home position sensor 240 is fixed at a position corresponding to a home position of the holding frame 226.
By using this apparatus, firstly, a position detection at a rough position is carried out. Concretely, a predetermined drive pulse is applied to the piezoelectric element 230, and the amount of displacement of the holding frame 226 is obtained by a position detection pulse from the MR sensor 234. The amount of displacement which is obtained is divided by the number of drive pulses required for this displacement, and the amount of displacement of the holding frame 226 per drive pulse is obtained. Based on the amount of displacement per drive pulse which is obtained, the number of drive pulses corresponding to the desired amount of displacement of the holding frame 226, are applied to the piezoelectric element 230, and the holding member 226 is positioned. Concretely, for instance, the holding frame 226 is moved once to the home position, and with this as a reference, the predetermined number of drive pulses are applied to the piezoelectric element 230, and the holding frame is made to be displaced to the desired position.
However, since a frictional force due to the attachment of the drive shaft 222 and the lens holding frame 226 is not constant in a movement stroke, there is a problem that the amount of movement of the lens holding frame 226 (moving body) per drive differs for each pulse. Consequently, only by applying the number of pulses based on the amount of movement per one drive which is calculated, an error between the amount of movement per drive which is calculated and the actual amount of movement per drive piles up, and it is not possible to carry out accurate positioning finally.